1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side light type, in which a point light source such as a light emitting diode (LED) is used as a light source, and more particularly to a spread illuminating apparatus suitable as a backlight for a liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display (LCD) panel characterized in having a small thickness does not emit light by itself and therefore needs an illuminating means for displaying images. A spread illuminating apparatus of side light type, which is structured such that a light source is disposed at a side surface (light entrance surface) of a light guide plate disposed under an LCD panel, is widely used as such an illuminating means, especially in the filed of small portable information devices, such as a mobile phone.
In order to achieve a high quality display on an LCD panel, it is always required for the spread illuminating apparatus to provide enhanced brightness uniformity, and various improvement approaches have been developed in order to meet the requirements. For example, a light emitting pattern composed of a plurality of prisms and disposed at the light reflection surface (bottom surface) of the light guide plate is structured such that the number of prisms per area gradually increases with an increase in distance from the light entrance surface of the light guide plate, whereby light is emitted substantially uniformly from the entire area of the light emitting surface (top surface) of the light guide plate regardless of the distance from the light emitting surface. Also, a light diffusing sheet to diffuse light emitted from the light guide plate to thereby make the emitted light uniform, and two prism sheets to adjust directivity characteristics with respect to two axial directions orthogonal to each other are disposed at the light emitting surface in a laminating manner, whereby illumination light is made further uniform.
When a point light such as an LED is used as a light source disposed at the light entrance surface of the light guide plate, a light introducing prism mechanism is generally disposed at the light entrance surface, whereby light emitted from the LED and introduced into the light guide plate is spread in the direction parallel to the light entrance surface of the light guide plate so that illumination light is available also at the corner portions of the light guide plate (refer to, for example, Japanese Patent Application Laid-Open No. 2002-42534).
Also, a light diffusing pattern composed of a plurality of prisms (ridge-and-groove structure) is disposed at the light emitting surface of the light guide plate, wherein the prisms extend in the direction substantially perpendicular to the light entrance surface of the light guide plate, whereby light traveling in the light guide plate is diffused in the direction parallel to the light entrance surface and illumination light is made further uniform (refer to, for example, Japanese Patent Application Laid-Open No. 2003-234004).
However, a spread illuminating apparatus, in which a light introducing prism mechanism is formed at a light entrance surface of a light guide plate, a light emitting pattern is formed at a light reflecting surface of the light guide plate, a light diffusing pattern is formed at a light emitting surface of the light guide plate, and a light diffusing sheet and two prism sheets are disposed at the light emitting surface, has the following problem. FIG. 10 shows such a conventional spread illuminating apparatus as described above, in which while illumination light is emitted from the light emitting surface of the light guide plate in a uniform manner on the whole, two bright lines are visibly present symmetrically to each other in the neighborhood of a light entrance surface 101a of a light guide plate 101, close to which an LED 102 is disposed. The bright lines are formed respectively along lines inclined about 50 degrees in both directions from an optical axis (an axis extending from the center of the LED 102 in the direction perpendicular to the light entrance surface 101a). If such bright lines can be well covered up by a frame-shaped sheet disposed at the light emitting surface of the light guide plate 101, the brightness uniformity is not disturbed thus raising no practical problem. However, in order to achieve a small spread illuminating apparatus with a narrowed frame in which an area covered by a light blocking sheet thus becoming unusable as an illumination region (ineffective area) is reduced for the purpose of meeting ever increasing demands for further downsizing the apparatus, the generation of bright lines which deteriorate the brightness uniformity must be suppressed.
The present invention has been made in light of the foregoing, and it is an object of the present invention to provide a spread illuminating apparatus of side light type, in which bright lines are suppressed from being generated in the neighborhood of a point light source such as an LED, and good brightness uniformity is achieved.
In order to accomplish the present invention, the present inventor attempted to study and clarify the mechanism for causing the generation of bright lines the present invention is to address. Based on the obtained findings about the mechanism for causing the generation of bright lines, the inventor made further investigations, and the present invention has been accomplished. So, for facilitating understanding of the present invention, the cause for generating bright lines in conventional spread illuminating apparatuses will be explained with reference to FIG. 10.
First, attention is paid to the behavior of light traveling in the light guide plate 101 in the optical axis direction. In the explanation below, the angle at which light travels in the optical axis direction is defined as 0 degree (hereinafter, the angle is referred to as “travel angle” or simply as “angle”). Light traveling with a travel angle of 0 degree is reflected or scattered at the light emitting pattern disposed at the bottom surface (light reflecting surface) of the light guide plate 101, and light reflected or scattered and incident on the top surface (the light emitting surface, exactly the surface of the light disusing pattern) of the light guide plate 101 at an angle smaller than a critical angle θc of the material of the light guide plate 101 is adapted to pass through the top surface of the light guide plate 101 and becomes illumination light.
Then, the travel angle is sequentially changed with the light flux kept virtually constant from 0 degree until light is caused to travel in the direction parallel to the light entrance surface 101a. While this change is seen from above the top surface of the light guide plate 101, it comes to a point where light is incident on the prism of the light diffusing pattern (ridge-and-groove-structure) formed at the top surface of the light guide plate 101 at the critical angle θc with respect to the ridge direction of the prism. Since the prism of the light diffusing pattern is formed to extend in the direction perpendicular to the light entrance surface 101a, the travel angle of the light incident on the prism at the critical angle θc is (90−θc) degrees. Light with a travel angle of less than (90−θc) degrees has its angle changed by reflection or scatter at the light emitting pattern like the light with a travel angle of 0 degree until the travel angle reaches or exceeds (90−θc) degrees, only after which the light is allowed to exit the light guide plate 101 from the top surface as illumination light. On the other hand, light which has a travel angle of greater than (90−θc) degrees already at the beginning is allowed to pass through the surface of the light diffusing pattern as illumination light without having the travel angle changed at the light emitting pattern.
Thus, light traveling in the direction centered on the optical axis (traveling through a region A defined between two broken lines shown in FIG. 10) has its incidence angle changed with respect to the top surface of the light guide plate 101 by means of the light emitting pattern formed at the bottom surface of the light guide plate 101 and thereby is adapted to exit the light guide plate 101 from the top surface (hereinafter, this emission mode is referred to as “light emission mode A”). On the other hand, light traveling in the direction defining a travel angle of greater than about (90−θc) degrees (traveling through a region B defined between each broken line and the light entrance surface 101a shown in FIG. 10) is adapted to exit the light guide plate 101 from the top surface without changing the travel angle at the light emitting pattern (hereinafter, this emission mode is referred to as “light emission mode B”). That is to say, the light emission mode differs on reaching a travel angle of (90−θc) degrees, and in this case, if lights are simultaneously emitted to travel in all the directions, brightness discontinuity is caused to appear along the direction of the angle of (90−θc) degrees. Specifically, when the light guide plate 101 is made of polycarbonate having a refractive index of 1.58, since the polycarbonate has a critical angle θc of 39.2 degrees with respect to air, the direction of (90−θc) degrees, along which the brightness discontinuity appears, is about 50.8 degrees. This direction substantially agrees with the direction along which a bright line visibly appeared in a research prototype.
The amount per unit area of light emitted from the top surface of the light guide plate 101 is compared between the light emission mode A and the light emission mode B, and it is shown that the amount emitted by the light emission mode A is smaller than the amount emitted by the light emission mode B. This means that the illumination light is brighter at the region B than at the region A. This is attributed to that the light emitting pattern for the light emission mode A is structured such that incidence angle relative to the top surface of the light guide plate 101 gradually decreases so that light is adapted to gradually exit from the top surface of the light guide plate 101. It becomes more likely that the brightness is higher at the region B especially when the light emitting pattern is formed such that a pattern density decreases with a decrease in distance to the light entrance surface 101a, in which case the brightness difference at the brightness discontinuity area becomes larger.
As is well known, the amount of light (light flux) traveling in the light guide plate 101 practically varies depending on the travel angle, specifically, decreases with an increase in travel angle. Consequently, the illumination light brightness decreases largely with a decrease in distance to the light entrance surface 101a while it departs from the brightness discontinuity area. So, it was assumed that observers view the brightness discontinuity area as a bright line because the brightness considerably decreases with a decrease in distance to the light entrance surface 101a while the brightness discontinuity area is generated.